转载于：
http://www.cocoachina.com/ios/20150409/11505.html
来源：Glow 技术团队博客
我们来看看apple为我们提供了哪些API来设置navigationBar的颜色。
首先想到的是最常用的[UINavigationBar appearance]，我们一般会在AppDelegate中使用它对navigationBar进行统一的设置。但是如果试一下，会发现在scrollViewDidScrollView中调用它并不能动态地改变navigationBar的颜色，原因可以看一下Apple的doc：
Use the UIAppearance protocol to get the appearance proxy for a class. You can customize the appearance of instances of a class by sending appearance modification messages to the class’s appearance proxy.
但是：
iOS applies appearance changes when a view enters a window, it doesn’t change the appearance of a view that’s already in a window. To change the appearance of a view that’s currently in a window, remove the view from the view hierarchy and then put it back.
所以变通一下，我们的思路很简单，参照Apple的实现，在navigationBar的view hierarchy中插入一个view，通过它来控制在navigationBar的backgroundColor。考虑到继承UINavigationBar使用起来会非常不便，我们决定用Category来实现，首先定义我们的category：
#import <objc/runtime.h>
@interface UINavigationBar (BackgroundColor)
- (void)lt_setBackgroundColor:(UIColor *)backgroundColor;
@end
实现：我们使用associatedObject将overlayView动态地绑定到UINavigationBar的instance上，当调用lt_setBackgroundColor的时候，我们只要更新这个overlayView就行啦～
@implementation UINavigationBar (BackgroundColor)
static char overlayKey;
- (UIView *)overlay
{ return objc_getAssociatedObject(self, &overlayKey);
}
- (void)setOverlay:(UIView *)overlay
{
objc_setAssociatedObject(self, &overlayKey, overlay, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}
- (void)lt_setBackgroundColor:(UIColor *)backgroundColor
{ if (!self.overlay) {
[self setBackgroundImage:[UIImage new] forBarMetrics:UIBarMetricsDefault];
[self setShadowImage:[UIImage new]]; // insert an overlay into the view hierarchy
self.overlay = [[UIView alloc] initWithFrame:CGRectMake(0, -20, [UIScreen mainScreen].bounds.size.width, 64)];
[self insertSubview:self.overlay atIndex:0];
}
self.overlay.backgroundColor = backgroundColor;
}
@end
我们就可以动态地修改UINavigationBar的backgroundColor了：
[self.navigationController.navigationBar lt_setBackgroundColor:[color colorWithAlphaComponent:alpha]];
